The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme). Each pixel in the printed image is derived from ink drops ejected from one or more ink nozzles. In recent years, inkjet printing has become increasing popular primarily due to its inexpensive and versatile nature. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents.
The Applicant has developed a range of pagewidth printheads. Pagewidth printheads have an elongate array of nozzles extending the printing width of the media substrate. These printheads are faster than traditional scanning printheads as the paper continuous feeds past the printhead which remains stationary. In contrast, scanning printheads traverse the page to print successive swathes as the paper is indexed through the printer.
The large number of nozzles in a pagewidth printhead generates much more heat than a corresponding scanning printhead. This requires pagewidth printheads to be ‘self cooling’ as complex and elaborate cooling systems would not be commercially practical. Self cooling is a process whereby heat generated in the ejection process is removed from the printhead by the ejected drops of ink. Without a build up of excessive heat, the theoretical maximum firing frequency of a self cooling printhead nozzle is only restricted by the ink refill rate of the nozzle.
Low energy droplet ejection is key to the Applicants printheads self cooling operation. Reducing the energy input to each nozzle, reduces the energy that the ejected drops need to remove in order to achieve self cooling operation. Thermal inkjet uses pulses of electrical current to raise the temperature of the heaters to the superheat limit of the ink, which is typically around 300° C. for water based ink. At this temperature a high pressure vapour bubble is formed on the heater surface and expansion of the bubble forces ink out of the nozzle. Reduced energy input in thermal inkjet can be achieved through careful attention to parasitic losses in the heater contacts. Careful attention must also be given to the reliability of the heater contact design.
The heater is a film of resistive material deposited by a lithographic process of the type well known and understood in the field semiconductor fabrication. When the film is deposited on a non-planar topography, the thickness of the film varies substantially. If the film is deposited over a substantially vertical step, the film thickness on the vertical surface of the step is typically ˜⅓ of the horizontal film thickness. A conductive strip of uniform width deposited over a vertical step will therefore have ˜3 times the current density in the vertical section with ˜9 times the volumetric heating rate (the heating rate is proportional to the square of current density). The temperature of relatively thin sections of film will far exceed 300° C. during the current pulse. This causes early failure due to, inter alia, oxidation and electro-migration.
One approach to avoid this is described in the Applicant's co-pending U.S. Ser. No. 11/246,687 filed Oct. 11, 2005, the contents of which are incorporated herein by cross reference. The current density in regions with non planar topography is reduced by making the width of the conductive strip much wider in that section. The additional width compensates for areas of reduced thickness and current density remains at safe levels.
Unfortunately, the electrical current funnels from the (laterally) wide contacts of the heater to the laterally much narrower resistive element that forms the vapour bubble. If the funnelling is done over a short distance, spikes in current density and hot spots can arise at or near the ends of the resistive elements, again causing early failure. Funnelling over a longer distance avoids hot spots but the parasitic resistance of the contact (i.e. non-bubble forming) portion of the heater increases, resulting in decreased efficiency.
Another technique for addressing excess current density is described in US patent publication 2008/0259,131 assigned to Lexmark International Inc. An additional low resistivity layer is deposited on top of the resistive thin film to ‘short out’ areas of the heater contacts deposited over non-planar topography. Volumetric heating rate is proportional to resistivity and hence the contacts sections stay relatively cool. The parasitic resistance and waste heat are low, as all but the active element of the heater is shorted by the low resistivity layer.
Unfortunately, both the resistive heater film and the low resistivity layer must be coated with an insulating layer to prevent contact with ink, or a corrosive galvanic cell will form (two dissimilar metals in contact in the presence of an electrolyte). Also, the traditional material for the low resistivity layer (aluminium) chemically corrodes if exposed to ink.
Coating with insulating layers increases the thermal mass that must be heated to the superheat limit to form a bubble, so this coating will increase the energy required to jet ink. As such, insulating coatings are contrary to energy efficient droplet ejection and therefore counter to self cooling operation.
A second drawback relates to patterning the low resistivity layer without damaging the underlying heater material film. Dry etches are preferred in most semiconductor fabrication facilities, but dry etches with suitable selectivity between the two materials, both likely to contain aluminium, are unlikely to exist. Finding a wet etch that can etch the low resistivity layer without etching the resistive thin film is likely to be easier, but that would impose significant constraints on the selection of the heater film material. These selection constraints may be contrary to the goal of self cooling, which requires thin film materials with particular properties, such as very high oxidation resistance.